Elsevier

Desalination

Volume 537, 1 September 2022, 115848
Desalination

Performance investigation of a novel solar direct-drive sweeping gas membrane distillation system with a multi-surface concentrator

https://doi.org/10.1016/j.desal.2022.115848Get rights and content

Highlights

  • A novel bionic solar direct-drive membrane desalination system was developed.

  • A high-efficiency multi-surface concentrator for the small-scale system was designed.

  • The mathematical modelling of the system was established.

  • Effects of operation parameters and the concentrator on the performance were studied.

  • The water yield per collector area is 5.1 kg/m2/d with the payback time of 3.4 years.

Abstract

Solar membrane distillation is a promising solution to overcome water shortage. However, developing a portable and highly efficient solar membrane distillation system is still a complex task. This work proposes a novel solar direct-drive sweeping gas membrane distillation with a multi-surface concentrator for families in rural areas. The optimized concentrator greatly improved the working temperature of the feed and the membrane distillation with sweeping gas configuration reduces the energy loss which both improve the performance of the system. Based on the optical simulation, a high optical efficiency optimized concentrator (over 70% within the incident angle of 20°) was designed. By conducting outdoor experiments and mathematical simulation, the effects of operating parameters and the concentrator on the system's performance were investigated. The experimental results indicate that the highest energy efficiency and the water production per collector area of the system with the concentrator reach 0.68 and 5.1 kg/m2/d. Simulation results show that the water yield increases with the increase of the irradiance and inlet air temperature and decreases with the seawater salinity increasing. The economic analysis suggests that the levelized cost of water and the payback period are $5.9/ton and 3.4 years respectively.

Introduction

Safe and readily accessible water is crucial for elevating the health conditions of human beings and accelerating social developments. However, about half of the world's population will be living in water impoverished area by 2025 based on the report of World Health Organization [1]. So, it is necessary to provide clean drinking water for people, especially in the impoverished areas. However, at present, distillation or water-treatment systems is usually large-scale, complex, and high cost distillation or sewage disposal processes, such as thermal-based multi-stage flash (MSF) and multi-effect distillation (MED), which also consume significant amount of primary energy (3-6 kW·h electric energy per m3 of freshwater produced and 50-70 kW·h thermal energy (fossil fuels) per m3 of freshwater produced respectively) [2], [3]. Such energy-intensive and expensive distillation processes are highly unlikely to be viable for people living in isolated islands or in rural areas) [4]. Besides, thermal-based multi-stage flash and multi-effect distillation usually require vast installation space which reduces their miniaturization and compactness [5].Therefore, a small-scale or a stand-alone distillation system driven by renewable energy resources, is urgently needed. Membrane distillation (MD) belongs to the temperature-driven membrane distillation technology which can operate under lower temperature (30 °C–90 °C) [6], [7]. In a MD system, low-grade energy could be utilized as the heat source to convert the untreated water or seawater directly into water vapor. Then the vapor penetrates through a hydrophobic and porous membrane to a cold permeate side due to the differences in partial vapor pressure, as a result of the temperature gradient across the membrane [8]. Based on the method of collecting the vapor at permeate side, there are four membrane distillation configurations: vacuum membrane distillation (VMD), direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD) and sweeping gas membrane distillation (SGMD). Compared with other desalination systems (i.e., MSF and MED), membrane desalination can avoid potential fouling and corrosion [9]. Furthermore it is suitable for the compact design because the flexible membrane can extend the interfacial area between the feed and permeate [10]. Solar energy is an inexhaustible clean low-grand energy which can be obtained anywhere on earth [11]. Therefore, solar drive membrane distillation is one of the most promising independent solar distillation technologies for a small-scale or stand-alone distillation system and has been widely studied in recent years [12], [13], [14], [15], [16].

According to whether the solar energy directly drives the membrane distillation module, solar drive membrane distillation systems can be divided into two types: (1) Conventional solar membrane distillation (CSMD) (Fig. 1(a)); (2) Solar direct-drive membrane distillation (SDMD) (Fig. 1(b, c)).

In the conventional solar membrane distillation system shown in Fig. 1(a), the solar energy is converted into heat by the collector (i.e., evacuated solar tube (EST) [17], [18], [19], [20], flat plate collector [21], solar pond [22] or solar still [23]), and then is stored in an energy storage tank. Following that, the feed is heated by the heat exchanger which absorbs heat from the energy storage tank. Finally, the high-temperature feed will evaporate in the membrane distillation module. However, for the conventional solar membrane distillation, their structures require a lot of pipes and higher power pumps to connect the photothermal conversion module, heat exchanging module and membrane distillation module which causes the system to be more complicated, high maintenance cost and low energy efficiency [24]. Therefore, these disadvantages make the conventional solar membrane distillation not suitable for families in rural areas.

To overcome the shortcomings of the conventional membrane distillation, solar direct-drive membrane distillation system is proposed as shown in Fig. 1(b) where the solar collector integrated with the membrane distillation module was used to directly heat the feed. Chen al at. [25] proposed a system with the DCMD configuration where a 0.01 mm blackened aluminum plate was used as a solar absorber to directly heat the feed. The indoor experimental results for this design showed that, the high feed flux can improve water production. Besides a mathematical model of the DCMD system was proposed to study its performances. However, the feed temperature is still lower than 50 °C even under high irradiance of 1100 W/m2. Bamasag et al. [26] integrated an evacuated solar tube with DCMD module to form a solar direct-drive membrane desalination system. To increase the feed temperature, extra evacuated solar tubes were used to preheat the feed. The outdoor experimental results showed that water production was 0.594 L from 11:00 to17:30, with total solar radiation of 6.1 kWh/m2. However, the energy efficiency of the system was calculated to be low, only around 0.24. The reason is that in the DCMD, more sensible heat in the feed was lost to the permeate because the low-temperature permeate was placed in direct contact with the high-temperature feed through the membrane. Therefore, the energy efficiency of this system was expected to be low [27]. Li et al. [28] inserted four capillary hollow fibers into evacuated solar tubes based on the VMD configuration. When the hollow fibers worked at the vacuum pressure of 10 kPa, the highest feed temperature of the system was between ~50–65 °C without any other auxiliary heating systems and the energy efficiency was between 0.36 and 0.46. For improving achieving the better performance of vacuum membrane distillation, Bamasag et al. [29] proposed a new solar direct-drive VMD with agitation device. The air pump and submersible water pump were used to stir the feed in the EST to create turbulence which can reduce the influent of concentration and temperature polarizations. The feed temperature of the system was between ~40–70 °C and the day efficiency of the system in day can reach 0.48. However, these systems based on VMD usually took the morning to preheat the seawater. For example, Li’s system [28] initially took more than 2 h (9:40–12:00) to preheat the feed from 20 °C to 65 °C at the irradiance of approximately 850 W/m2 and then the vacuum pump was switched on to produce the freshwater. Besides, for the VMD, the system has to be operated under vacuum conditions, which poses challenges for proper sealing, as well as introducing pore wetting risk [30]. Finally, the vacuum pump also will increase the capital and maintenance cost.

Therefore, some challenges still exist (low feed temperature, low energy efficiency, no portability) in the solar direct-drive membrane distillation with solar collector. To overcome these issues mentioned earlier, the primary aim of the work is to investigate a novel portable solar direct-drive membrane distillation system based on the SGMD configuration for families in rural areas or islands. Secondly, to the best of our knowledge, there is also no solar direct-drive membrane distillation with concentrator which is researched in indoor or outdoor [10] and no experimental or numerical research on the solar direct-drive SGMD system. So, this paper is the first attempt to design a suitable concentrator for a small-scale solar distillation system and study and predict the influences of various operating parameters and the concentrator on the performance of the system based on outdoor experiments and mathematical modelling of the solar driven SGMD system.

Compared with VMD, the system with SGMD working at atmospheric pressure configuration has a low sealing requirement and no pore wetting risk. SGMD also has lower thermal polarization and lower thermal loss compared to the DCMD which is important to improve energy efficiency [30]. In addition, by developing a highly efficient multi-surface concentrator for the system, the feed temperature can reach over 80 °C which improves the system's energy efficiency; the solar drive membrane distillation system with the concentrator also can eliminate additional preheating devices (additional collector tubes or electric heating) which make the system more keep compact structure, reliable operation, and low cost. At last, because low electricity consumption of pump and fan, a small photovoltaic panel can drive the system with SGMD. So, the whole device can be driven by solar energy. In a word, the whole system is easy to operate and carry, and has low equipment investment and low maintenance cost which is very suitable for families in remote areas.

Section snippets

Working principle of the system

a novel solar direct-drive membrane humidifier is proposed, where the SGMD module and the evacuated solar tube with a new multi-surface concentrator (MSC) are directly integrated together as shown in Fig. 2(a). For a humidification system, the higher the feed temperature, the higher the evaporation efficiency. So, a new MSC is developed to increase the light absorbing area which increases the feed temperature to over 80 °C without adding other complicated auxiliary preheating systems. The

Material

Fabrication of the hydrophobic steel bracket which is used to support membrane tube: the steel bracket was ultrasonically washed with deionized water and ethanol and then dried at 45 °C in a drying oven. D20 hydrophobic nano-coating (Green tree company) was evenly sprayed on the steel bracket with a spray gun (3–4 times). Then hydrophobically treated steel bracket was heated in a 150 °C drying oven for 30 min. Finally, the steel bracket was cooled at room temperature.

Characterization

A field emission scanning

Material characteristics

As illustrated in Fig. 6(a), a type of microporous membrane tube made from PTFE was used. The scanning electron microscope (SEM) of the membrane tube (Fig. 6(b)) shows that it is a porous membrane, and its average pore size is about 1 μm which ensures only vapor can pass through the membrane. Besides, the membrane is superhydrophobic and its water contact angle is 130° (Fig. 6(c)), which reduces the pore wetting risk of the membrane. The other specific parameters of the PTFE membrane tube are

Conclusions

A novel solar direct-drive membrane distillation system was proposed in this paper, as a solution to provide clean drinking water for families in rural areas or islands. Firstly, the MSC with a high light receiving rate was designed, optically optimized and constructed. Then effects of the operating parameters and the concentrator on the performance of the proposed system were analyzed based on experimental testing and numerical simulations. Finally, performance comparison with other systems

CRediT authorship contribution statement

Yunsheng Zhao: Investigation, Software, Writing – original draft, Visualization. Xiangjie Chen: Writing – review & editing, Visualization. Omar Ramadan: Writing – review & editing, Visualization. Hongyu Bai: Resources, Data curation. Yuehong Su: Resources, Data curation. Hongfei Zheng: Conceptualization, Methodology, Funding acquisition, Supervision. Saffa Riffat: Conceptualization, Methodology, Funding acquisition, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the gracious support of the National Natural Science Foundation of China (No. 51976013) and the China scholarship council UK-China Joint Research and Innovation Partnership Fund (Ph.D. Placement Program).

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